Generally speaking, a mode-locked laser refers to a laser where all the longitudinal modes in the laser cavity are phase locked or mode-locked, which is a technique that can be used to produce extremely short duration laser pulses on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). Because high peak power can be achieved due to the extremely short pulse duration, mode-locked lasers can be used for a variety of applications, including materials processing, spectroscopy, sensing, medicine, and light generation through optical nonlinearity.
Mode-locked fiber lasers are a specific type of mode-locked lasers comprising a doped fiber as a gain medium. Typically, a gain fiber comprising a length of several meters is used because of the relatively low gain per unit length of doped silica fiber. Furthermore, when a gain fiber comprising a length of several meters is used, the repetition rate of a pulse train of the mode-locked fiber laser is limited to approximately 100 MHz.